Lubricating oil is the life blood of oil wetted machinery. In-service oil analysis can provide information about machine wear condition, lubricant contamination as well as lubricant condition (Figure 1). Reliability engineers and maintenance professionals can make maintenance decisions based on diagnoses of oil analysis results. Machine wear analysis is essentially the analysis of particles in the oil. Machine wear can be classified into adhesive (sliding) wear, abrasive (cutting) wear, fatigue wear, and corrosive wear.

A full suite of wear particle analyses includes measure of particle count and distribution, particle shape and morphology, wear metal and alloy elements and the presence of large ferrous particles. Over the years different techniques and instruments have been deployed in the field or in commercial laboratories but essentially they all provide the information mentioned above.

Even though all wear particle analysis techniques can be applied to different types of assets, specific issues are different for industrial rotating machines than from rotating engines. Reciprocal engines tend to generate fine wear particles and engine oils can be dark due to soot (nano-sized carbon particles as combustion byproduct). Elemental analysis is primary or sometimes the only analysis performed in a commercial oil lab that assesses engine wear conditions because concentrations of different wear metal elements indicate wear severity of moving parts in an engine. Slower moving rotating machines, such as gearboxes, tend to generate larger wear particles.

As most of them are made of steel, large ferrous particle analysis is fairly common and easy to perform on oils from these assets. Contaminants in oil can be in solid or liquid form. Solid contaminants such as sand and dirt are commonly monitored by particle counting and sizing techniques. Liquid contaminant for industrial rotating machines is mostly water. For diesel or gas engines, however, it can be water, coolant or fuel. A fairly common fluid contaminant across all asset types is the accidental or purposeful use of an incorrect lubricant when topping off old oil. All contaminants can significantly reduce the useful life of the oil and increase machine wear. They need to be prevented proactively with proper seals and filtration systems and they need to be monitored regularly. Monitoring lubricant degradation helps one decide if the oil is no long fit for use and if it needs to be changed. One key oil property is viscosity.

The viscosity is typically measured at 40C for rotating machines and at 100C for engines. For oil in rotating machines, oxidation and acidity of the oil (Total Acid Number or TAN) are monitored to determine if the oil is still fit for use and to prevent corrosion. For engine oils, oxidation, nitration, sulfation and total alkaline additive reserve in oil (Total Base Number or TBN) are monitored. For natural gas engine oils, TAN is monitored along with other engine oil parameters.